As the demand for higher resolution cathode-ray tube performance continues, electron guns capable of smaller electron beam spot size performance at both high and low currents are being designed. As the spot size is reduced, the problem of moire fringes occurs at low currents due to the interaction of the scan line pattern with the pattern of the mask holes. Moire is a woodgrain pattern that is visible on a tube screen. Previous efforts to eliminate moire have included optimizing the scan line pattern in relation to the mask hole pattern through reduced vertical aperture spacing, rotated mask holes, and random mask hole patterns. The reduced vertical aperture spacing is, in principle, a solution to the moire problem. However, from practical considerations of mask strength and manufacturability, masks having small enough vertical aperture spacings are not practical to make for large screen tubes. The technique of rotated mask holes or random hole patterns can also reduce moire, but introduces other problems, such as a "grainy" screen appearance, which is just as objectionable as the moire itself.
It is, therefore, desirable to have an electron gun design in which high current electron beam spot sizes are reduced for improved resolution, but the low current spot size is kept above a minimum value to prevent moire fringes. Because it is difficult to tailor the design of a gun to have a small high current spot size while maintaining a specified minimum low current spot size in a standard gun design, the present invention provides a system incorporating an electron gun with improved spot size at all current levels, by utilizing a low current dynamic spot size control.